1. End Show Slide 1 of 24 Biology Variation Mr. Karns

2. End Show Slide 2 of 24 16-1 Genes and Variation

3. End Show 16-1 Genes and Variation Slide 3 of 24 How Common Is Genetic Variation? Many genes have at least two forms, or alleles. All organisms have genetic variation that is “invisible” because it involves small differences in biochemical processes. An individual organism is heterozygous for many genes.

4. End Show 16-1 Genes and Variation Slide 4 of 24 Variation and Gene Pools Genetic variation is studied in populations. A population is a group of individuals of the same species that interbreed. A gene pool consists of all genes, including all the different alleles, that are present in a population.

5. End Show 16-1 Genes and Variation Slide 5 of 24 Variation and Gene Pools The relative frequency of an allele is the number of times the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur. Relative frequency is often expressed as a percentage.

6. End Show 16-1 Genes and Variation Slide 6 of 24 Variation and Gene Pools Gene Pool for Fur Color in Mice Sample Population Frequency of Alleles allele for brown fur allele for black fur

7. End Show 16-1 Genes and Variation Slide 7 of 24 How is evolution defined in genetic terms? Variation and Gene Pools

8. End Show 16-1 Genes and Variation Slide 8 of 24 Variation and Gene Pools In genetic terms, evolution is any change in the relative frequency of alleles in a population.

9. End Show 16-1 Genes and Variation Slide 9 of 24 What are the main sources of heritable variation in a population? Sources of Genetic Variation

10. End Show 16-1 Genes and Variation Slide 10 of 24 Sources of Genetic Variation The two main sources of genetic variation are mutations and the genetic shuffling that results from sexual reproduction.

11. End Show 16-1 Genes and Variation Slide 11 of 24 Sources of Genetic Variation Mutations A mutation is any change in a sequence of DNA. Mutations occur because of mistakes in DNA replication or as a result of radiation or chemicals in the environment. Mutations do not always affect an organism’s phenotype.

12. End Show 16-1 Genes and Variation Slide 12 of 24 Sources of Genetic Variation Gene Shuffling Most heritable differences are due to gene shuffling. Crossing-over increases the number of genotypes that can appear in offspring. Sexual reproduction produces different phenotypes, but it does not change the relative frequency of alleles in a population.

13. End Show 16-1 Genes and Variation Slide 13 of 24 What determines the numbers of phenotypes for a given trait? Single-Gene and Polygenic Traits

14. End Show 16-1 Genes and Variation Slide 14 of 24 Single-Gene and Polygenic Traits The number of phenotypes produced for a given trait depends on how many genes control the trait.

15. End Show 16-1 Genes and Variation Slide 15 of 24 Single-Gene and Polygenic Traits A single-gene trait is controlled by one gene that has two alleles. Variation in this gene leads to only two possible phenotypes.

16. End Show 16-1 Genes and Variation Slide 16 of 24 Single-Gene and Polygenic Trait The allele for a widow’s peak is dominant over the allele for a hairline with no peak. However, the presence of a widow’s peak may be less common in a population. In real populations, phenotypic ratios are determined by the frequency of alleles as well as by whether the alleles are dominant or recessive.

17. End Show 16-1 Genes and Variation Slide 17 of 24 Single-Gene and Polygenic Trait Many traits are controlled by two or more genes and are called polygenic traits. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait.

18. End Show 16-1 Genes and Variation Slide 18 of 24 Single-Gene and Polygenic Trait A bell-shaped curve is typical of polygenic traits. A bell-shaped curve is also called normal distribution.

19. End Show - or - Continue to: Click to Launch: Slide 19 of 24 16-1

20. End Show Slide 20 of 24 16-1 Which of the following statements is TRUE? a.The relative frequency of an allele is not related to whether the allele is dominant or recessive. b.Mutations always affect an organism's phenotype. c.Crossing over decreases the number of different genotypes that appear in an offspring. d.Evolution does not affect the frequency of genes in a gene pool.

21. End Show Slide 21 of 24 16-1 Most inheritable differences are a result of a.gene shuffling. b.frequency of alleles. c.mutations. d.DNA replication.

22. End Show Slide 22 of 24 16-1 The main sources of inherited variation are a.gene shuffling and mutations. b.gene pools and frequencies. c.single-gene and polygenic traits. d.genotypes and phenotypes.

23. End Show Slide 23 of 24 16-1 A widow's peak in humans is an example of a(an) a.invariable trait. b.single-gene trait. c.polygenic trait. d.mutation.

24. End Show Slide 24 of 24 16-1 A graph of the length of the little finger on the left hand versus the number of people having fingers of a particular length is a bell-shaped curve. This indicates that finger length is a a.single-gene trait. b.polygenic trait. c.randomly inherited trait. d.strongly selected trait.

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